A secondary radar (also known as an interrogator radar, surveillance radar or SSR—the acronym standing for the expression “Secondary Surveillance Radar”) is a device which dispatches a train of electromagnetic pulses containing an interrogation message. The interrogation message is intended for transponders (also known as radar responders) installed in carrier vehicles such as aircraft or ships. When a transponder is in the emission lobe of the secondary radar, it receives the interrogation message. It emits in return a train of electromagnetic pulses containing a response message. The response message, intended for the secondary radar, can contain data identifying the carrier vehicle.
The interrogation and response messages are coded according to determined protocols. In the field of civil aviation, protocols called modes A, B, C and D are conventionally used for air surveillance. These protocols are defined by the standard of the ICAO (the acronym standing for “International Civil Aviation Organization”). For military surveillance, protocols called modes 1, 2, 3 and 4 are used. The mode A protocol is identical to the mode 3 protocol. Modes 3/A and C are used for air traffic control.
In mode A, the aircraft (carrier vehicles) are identified by a discrete code. A code of four digits that are strictly less than 8 (coded on 12 bits) is assigned to each aircraft, these codes being reassignable. The number of different codes is 4096. There may be a conflict if two aircraft with the same code are situated simultaneously in the emission lobe of a secondary radar.
With the increase in air traffic, the conventional protocol of secondary radars is no longer suitable:                the risk of a conflict is too significant;        responses of two aircraft situated at one and the same distance from the secondary radar may overlap (synchronous overlap); and        a secondary radar may receive responses from an aircraft, made in response to the interrogations of another secondary radar (asynchronous responses or “fruit”).        
In order to solve these problems, the ICAO has defined a protocol called mode S. In this protocol, the identification codes are coded on 24 bits, thereby making more than 16 million different identification codes. Moreover, mode S relies on a procedure of selective calling which consists in interrogating a single aircraft (fully identified) at a time. Hereinafter in the description, the conventional interrogations and responses will be designated by SSR interrogations and responses.
The implementation of the mode S protocol requires the replacement of existing transponders with transponders of a new type. The existing transponders being unable to be replaced simultaneously, the new secondary radars (capable of interrogating in mode S) are compatible with conventional transponders. For this purpose, the mode S secondary radars also interrogate on conventional modes (SSR interrogations) between mode S periods.
The shape of the pulses of the mode S responses being identical to those of the SSR responses, some of the pulses forming a mode S response may be regarded wrongly as an SSR response by a secondary radar. This can lead to false detections or to saturation of the processings in a secondary radar operating in conventional mode. It may be desirable then, when seeking to detect SSR responses, to filter the pulses received during the whole duration of a mode S response.
However, a mode S response may overlap SSR responses, even if the aircraft responding in conventional mode are situated at different distances. Specifically, the mode S interrogations and responses are much longer than the SSR interrogations and responses. By filtering the pulses during the whole duration of a mode S response, SSR responses are also filtered.